Incident to many marine operations is a need to transport liquids or gases through a subsea flowline from an offshore location to a more readily accessible location. For example, in the oil and gas industry, many offshore production systems have production risers extending from the ocean floor to the production platform. This is particularly common with fixed platforms and tensioned leg platforms. To transport the liquid or gas produced by way of subsea flowlines, the flowlines are preferably connected to the riser at or near the ocean floor.
A particular problem arises when such connections are to be made in relatively deep waters, for example 300 feet or more. At such depths it is difficult, and often impossible, to use divers to aid in the final alignment and connection of the flowline with the riser. Thus, in deeper waters it is usually necessary to make the final alignments and connections from a remote location.
One method used today for remotely aligning and connecting the flowline with the riser, is disclosed in U.S. Pat. No. 3,690,112 which issued Sept. 12, 1972 to Pattison et al. This method employs a pipe guide, a mating receiver, a riser guide and a power driven pull-in cable. The pipe guide is a sled-like guide connected to the lead end of the flowline. The lead end of the flowline is curved so that the mouth of the flowline is directed about 90.degree. from the direction of the portion of the flowline away from the lead end. The flowline is connected to the pipe guide so that the curved portion points in an upwardly direction. The pipe guide, with the flowline, is moved by the pull-in cable to the structural guide which is fixedly attached to an offshore structure. The structural guide has at least one mating element and the pipe guide has the counterpart. The mating element is a female mating receiver that is formed to receive the male counterpart of the pipe guide. Once the pipe guide connects with the structural guide the flowline is in position for final connection to the riser. The riser is directed into position for connection to the flowline through a tubular riser guide which is also fixedly attached to the offshore structure. Once both the riser and the flowline are in position, the final connection is made.
One problem with the method disclosed above arises when the flowline and the pipe guide are not directly in line with the structural guide. As the pull-in cable draws the pipe guide toward the mating receiver, the flowline is forced out of its relaxed position as it is moved into direct alignment with the mating receiver. This puts the flowline into a stressed condition, creating an increased risk for future cracks or leaks. Furthermore, in order to move the flowline into direct alignment a stronger power source and pull-in cable are needed than would be needed if the flowline would be in its relaxed state when coupled with the structural guide.
Secondly, since the lead end of the flowline and the riser must have the same angular orientation, the method described above requires precise orientation of the flowline within the pipe guide. Slight variations in the curvature of the lead end of the flowline or in the angle of orientation of the flowline within the pipe guide can have a significant impact on the final alignment, making a remote connection quite difficult.
A second method for pulling a flowline to a subsea structure and remotely aligning it with a riser is disclosed in U.S. Pat. No. 4,591,292 which issued May 27, 1986 to Stevens, et al. By this method, the pull-in cable pulls the flowline to a rotatable alignment receiver. Once the flowline connects to the receiver, an electrical or hydraulic motor rotates the receiver which in turn adjusts the position and angular orientation of the flowline head for final connection with a riser.
Using the method of Stevens' et al., the flowline may be forced out of its relaxed state when it is connected to the alignment receiver if the horizontal orientation of the flowline is not in direct line with the alignment receiver. Once connected to the receiver, the lead end of the flowline is twisted or turned into position with the riser by the receiver. This increases the stress on the flowline and on the connection between the flowline and riser, thereby creating a greater risk of cracking or future leakage, in addition to increasing the complexity and number functions of the receiver.
It would be desirable to develop a method and apparatus to accurately align a subsea flowline with an offshore structure for connection thereto, from a remote location, that does not cause stress to the flowline and that allows for angular variations or positioning inaccuracies in the orientation of the lead end of the flowline, with a minimal number of mechanical connections and movement functions of the receiver.